1. Field of the Invention
The present invention relates to an optical recording medium where a light beam is focused on the recording medium and information is reproduced by detecting light reflected from the recording medium. Particularly, the present invention relates to an optical recording medium having dual information surfaces.
2. Description of the Related Art
In recent years, optical recording media have become more and more important as a means for storing sound information data, image information data, and various information apparatus data because they can store and reproduce a large amount of data. There are still requirements for further increasing the capacity of the optical recording media and reducing the size of optical recording/reproducing apparatuses. In order to satisfy these requirements, the storage capacity of the optical recording media needs to be further increased.
Compact disks (CDs) having one information surface, for example, are known as a conventional read-only optical recording medium. The CD includes a spiral information track composed of convex and concave portions (pits) formed on a surface of a disk-shaped resin substrate with a thickness of 1.2 mm. A reflection film made of aluminum and the like and a protection film are formed on the resultant information surface of the substrate by sputtering and the like. An identification label is then printed on the protection film.
The storage capacity of such a CD is small because the CD has only one information surface. In order to increase the storage capacity, a recording medium where two disks are adhered together, such as a 5" magneto-optical (MO) disk, has been commercialized. The 5" (MO) disk is classified into two types; a disk having one information surface (one-sided disk) and a disk having two information surfaces (double-sided disk). The one-sided disk includes a spiral guide groove composed of convex and concave portions formed on a surface of a disk-shaped resin substrate with a thickness of 1.2 mm. A dielectric film, a magneto-optical recording material film, another dielectric film, and a reflection film made of aluminum and the like are formed in this order on the resultant information surface of the substrate by sputtering and the like. Another resin substrate with a thickness of 1.2 mm is then adhered to the reflection film. The double-sided disk includes a spiral guide groove composed of convex and concave portions formed on a surface of a disk-shaped resin substrate with a thickness of 1.2 mm. A dielectric film, a magneto-optical recording material film, another dielectric film, and a reflection film made of aluminum and the like are formed in this order on the resultant information surface of the substrate by sputtering and the like. The thus-fabricated disk is adhered with another disk fabricated in the same manner. Conventional 5" MO disk recording/reproducing apparatuses are designed to receive both the one-sided disk and the double-sided disk to accomplish the recording and reproduction. The user can select the one-sided disk when information to be recorded is small or the double-sided disk when it is large. The 5" MO disk apparatuses are generally provided with only one optical head. Accordingly, when the double-sided disk is used, the disk needs to be taken out and turned over to continue the recording or reproduction.
In general, the information density of a recording medium is determined by the pitch of an information track and the information density in the tracking direction, i.e., the information linear density. In order to increase the information density of the recording medium, the track pitch should be small, while the linear density should be large. In recent years, there have been studies to increase the density of the optical recording medium by reducing the thickness of the substrate to 0.6 mm, for example, to reduce the aberration of a light beam passing through the substrate due to a tilt of the disk.
However, the above conventional techniques have the following problems. In the case of the conventional double-sided optical recording medium, if both the top and bottom surfaces of the recording medium are illuminated with light beams so as to record information or reproduce recorded information, little space is left on the surfaces of the recording medium for printing an identification label. This is inconvenient for handling the recording medium. Also, when the conventional double-sided optical recording medium is used for an optical reproduction apparatus having only one optical head, the optical recording medium needs to be taken out from the apparatus and turned over to continue the reproduction. In order to continue the reproduction automatically, two optical heads disposed above and below the recording medium are required. An apparatus having two optical heads is large in size and its cost is high.
Another problem is that when a new optical recording medium thinner than the conventional optical recording media is commercialized to increase the density of the recording medium, such a new optical recording medium is not compatible with the conventional recording/reproduction apparatus.